Due to high water consumption for irrigation of western Istria during the summer and for flood protection, the preliminary design of a dam and reservoir “Momjan” was made. This thesis defines the most important parameters for dam design such as reservoir size, location of the dam profile and type of the dam. Analyzing hydrological, geological and seismic data of the location and other factors important for selection of type of the dam, the rockfill dam with rip-rap and clay core was chosen. In addition to the characteristics of the dam and reservoir, in thesis are also determined the types of ancillary structures with associated dimensions. Two versions of structures for evacuation of high waters are made, first in the form of morning glory spillway which connects to the bottom outlet, and second with side channel spillway along the dam. Due to the easy way for foundation and construction, the side channel spillway was selected. As ancillary structures for the dam are provided side channel spillway, bottom outlet with lock chamber, control gallery and water-intake structure. Hydrological data used for the design are taken from the paper dealing with regulation of the Argila stream.